This invention relates generally high speed digital telecommunication systems. The invention more particularly relates to retiming circuits and methods for SONET (synchronous optical network) signals.
The telecommunications network servicing the United States and the rest of the world is presently evolving from analog transmission to digital transmission with ever-increasing bandwidth requirements. Fiber optic cable has proved to be a valuable tool of such evolution, replacing copper cable in nearly every application from large trunks to subscriber distribution plants. Fiber optic cable is capable of carrying much more information than copper with lower attenuation.
In attempting to accommodate the protocols, equipment, and cables of the past while providing for the direction of the future, various standards and system requirements relating to fiber optic cables have been adopted. In particular, the T1 Standards Committees of ANSI have provided a draft document ANSI T1.105-1988 dated Mar. 10, 1988 which sets forth specifications for a rate and format of signals which are to be used in optical interfaces. Additional details and requirements are set forth in Technical Advisory publications SR-TSY-000202, --000233, -000253, -000303 Issue 3 of Bell Communication Research (Bellcore). The provided specifications detail the SONET standard. SONET defines a hierarchy of multiplexing levels and standard protocols which allow efficient use of the wide bandwidth of fiber optic cable, while providing a means to merge lower level DS0 and DS1 signals in a common medium. In essence, SONET establishes a uniform, standardized transmission and signaling scheme which provides a synchronous transmission format that is compatible with all current and anticipated signal hierarchies. Because of the nature of fiber optics, expansion of bandwidth is easily accomplished.
A basic SONET signal, termed an STS-1 signal, is seen in FIG. 1. The SONET signal is a 51.84 Mhz, bit serial signal, having nine rows of ninety columns of eight bit bytes at a 125 microsecond frame rate. The first three columns of bytes in the SONET signal are termed the transport overhead (TOH) bytes and are used for various control purposes as indicated in FIG. 2. The remaining eighty-seven columns of bytes constitute the STS-1 Synchronous Payload Envelope (SPE) as seen in FIG. 3.
Turning to FIG. 2, it is seen that the first two bytes Al and A2 of the transport overhead are framing bytes which contain a specified framing pattern allowing synchronization of the basic SONET STS-1 signal. Three other bytes, H1, H2, and H3 form a pointer giving explicit information as to the location of the start of the SONET SPE (i.e. the "SPE phase"). The pointer bytes are required due to the fact that the position of the SPE is not fixed in time in the STS-1 frame, but is allowed to be displaced in time (i.e. a change in the location of the SPE in the frame over time constituted an SPE "phase" shift). Hence, as seen in FIG. 3, a single SPE is seen to typically straddle two consecutive STS-1 frames
As seen in FIG. 4 which sets forth the payload pointer coding, the pointer for the SPE is located in the last ten bits of the word formed by bytes H1 and H2. The pointer value is an offset value and designates the location after byte H3 of the first byte of the SPE. If the pointer value is zero (i.e. zero offset), the first byte of the SPE is located in the first byte position after the H3 byte. If the pointer value equals one, the SPE starts at the second byte past byte H3. The greatest value allowed for the pointer is seven hundred eighty-two (equal to 810 minus 27 minus 1; 810 bytes in the frame, less 27 bytes for the TOH, less one byte to find the final location). The value of seven hundred eighty-two indicates, as seen in FIG. 5 which shows byte locations, that the SPE starts at the last byte position before the Hl byte of the next STS-1 frame.
As indicated in FIGS. 4 and 5, during normal operation, two kinds of pointer adjustments are allowed. A negative stuff is utilized when the SPE being transported is running at a frequency higher than that of the STS-1 envelope (i.e. additional information must be inserted into the envelope), while a positive stuff is utilized where the SPE is running at a frequency slower than the STS-1 signal (i.e. stuff bytes are inserted into the envelope). Regardless, the SPE phase is moved by one byte, forward or backward in time (the SPE "phase" is retarded or advanced relative to the envelope itself).
While the SONET specifications provide standards which permit the SONET SPE to float within the STS-1 envelope, details of means for taking the SPE of one STS-1 envelope and inserting into the envelope of another STS-1 signal for transport are not known in the art. Clearly, then such means are needed.